1. Field of the Invention
This invention relates to a machine for drilling holes in a relatively brittle planar material, such as glass, in which each hole must be partly drilled from one side of the planar member and then finish drilled from the opposite side. More particularly, this invention relates to a machine of the foregoing character with improved means for axially aligning the drilling devices on opposite sides of the planar member being drilled and for controlling the operations of the drilling devices.
2. Description of the Prior Art
It is known in the prior art that any hole in a glass sheet or other relatively brittle planar member should be drilled partly from one side of the sheet by a first drilling device and then finish drilled from the opposite side by a second drilling device. See, for example, U.S. Pat. Nos. 3,124,016 (W. E. Reaser); 3,568,367 (D. O. Myers); 3,813,820 (C. W. Highberg et al.); 4,573,835 (R. Eckardt et al.); and 4,730,420 (H. Stratmann et al.). In the arrangements of the foregoing patents, each drilling device is positioned at the end of an arm, and each arm is movable either arcuately about a common axis or rectilinearly toward and away from the sheet being drilled to bring the drilling devices into and out of engagement with the sheet being drilled. Good drilling practice requires that the drilling devices be axially aligned during drilling, and to ensure such axial alignment it is important that the position of each drilling device be adjustable. Heretofore, such adjustment was provided by adjustably mounting each drilling device to the arm carrying such device, for example, by an adjustable dovetail fit between the drilling device and the arm. However, the drilling of glass sheets normally employs a cutting and cooling fluid which is directed at the location of the sheet being drilled, typically, in the case of a relatively large hole, by using a drill bit of annular configuration and by directing the fluid toward the sheet through the interior of the drill bit, and the fluid splashing which, thus, occurs in the drilling region tends to foul or contaminate the interengaging surfaces of the drilling device and the arm to which it is mounted, and complicates the making of adjustments of the drilling device relative to its arm.